The present invention relates to liquid droplet ejection systems and, more particularly, ink jet system and, even more particularly, to drop-on-demand ink jet systems.
Ink jet systems generally fall into two categories--continuous systems and drop-on-demand systems. Continuous inkjet systems operate by continuously ejecting droplets of ink, some of which are deflected by some suitable means prior to reaching the substrate being imprinted, allowing the undeflected drops to form the desired imprinting pattern. In drop-on-demand systems, drops are produced only when and where needed to help form the desired image on the substrate.
Drop-on-demand ink jet systems can, in turn, be divided into two major categories on the basis of the type of ink driver used. Most systems in use today are of the thermal bubble type wherein the ejection of ink droplets is effected through the boiling of the ink. Other drop-on-demand ink jet systems use piezoelectric crystals which change their planar dimensions in response to an applied voltage and thereby cause the ejection of a drop of ink from an adjoining ink chamber.
Typically, a piezoelectric crystal is bonded to a thin diaphragm which bounds a small chamber or cavity fill of ink or the piezoelectric crystal directly forms the cavity walls. Ink is fed to the chamber through an inlet opening and leaves the chamber through an outlet, typically a nozzle. When a voltage is applied to the piezoelectric crystal, the crystal attempts to change its planar dimensions and, because the crystal is securely connected to the diaphragm, the result is the bending of the diaphragm into the chamber. The bending of the diaphragm effectively reduces the volume of the chamber and causes ink to flow out of the chamber through both the inlet opening and the outlet nozzle. The fluid impedances of the inlet and outlet openings are such that a suitable amount of ink exits the outlet nozzle during the bending of the diaphragm. When the diaphragm returns to its rest position ink is drawn into the chamber so as to refill it so that it is ready to eject the next drop.
Thermal bubble systems, although highly desirable for a variety of applications, suffer from a number of disadvantages relative to piezoelectric crystal systems. For example, the useful life of a thermal bubble system print head is considerably shortened, primarily because of the stresses which are imposed on the resistor protecting layer by the collapsing of bubbles. In addition, because of the inherent nature of the boiling process, it is relatively difficult to precisely control the volume of the drop and its directionality. As a result, the produced dot quality on a substrate may be less than optimal.
Still another drawback of thermal bubble systems is related to the fact that the boiling of the ink is achieved at high temperatures, which calls for the use of inks which can tolerate such elevated temperatures without undergoing either mechanical or chemical degradation. As a result of this limitation, only a relatively small number of ink formulations, generally aqueous inks, can be used in thermal bubble systems.
These disadvantages are not present in piezoelectric crystal drivers, primarily because piezoelectric crystal drivers are not required to operate at elevated temperatures. Thus, piezoelectric crystal drivers are not subjected to large heat-induced stresses. For the same reason, piezoelectric crystal drivers can accommodate a much wider selection of inks. Furthermore, the shape, timing and duration of the ink driving pulse is more easily controlled. Finally, the operational life of a piezoelectric crystal driver, and hence of the print head, is much longer. The increased useful life of the piezoelectric crystal print head, as compared to the corresponding thermal bubble device, makes it more suitable for large, stationary and heavily used print heads.
Piezoelectric crystal drop-on-demand print heads have been the subject of much technological development. Some illustrative examples of such developments include U.S. Pat. Nos. 5,087,930 and 4,730,197, which are incorporated by reference in their entirety as if fully set forth herein and which disclose a construction having a series of stainless steel layers. The layers are of various thicknesses and include various openings and channels. The various layers are stacked and bonded together to form a suitable fluid inlet channel, pressure cavity, fluid outlet channel and orifice plate.
The systems disclosed in the above-referenced patents illustrate the use of a fluid inlet channel having a very small aperture, typically, 100 microns or less. The use of a very small aperture is dictated by the desirability of limiting the backflow from the ink cavity during ejection of a drop but is problematic in that the small aperture is susceptible to clogging during the bonding of layers as well as during normal operation of the print head.
The construction disclosed in the above-referenced patents requires the very accurate alignment of the various layers during manufacture, especially in the vicinity of the small apertures which form portions of the fluid path. Furthermore, the openings in the orifice plate which form the outlets of the various flow channels have sharp edges which could have adverse effects on the fluid mechanics of the system.
Additionally, the techniques used in forming the openings in the orifice plate, which typically include punching, chemical etching or laser drilling, require that the thickness of the orifice plate be equal to, or less than, the orifice diameter which is itself limited by resolution considerations to about 50 microns.
Finally, any air bubbles trapped inside the flow channel cannot easily be purged and, because the bubbles are compressible, their presence in the system can have detrimental effects on system performance.